A radio communication system with two-way communication between base stations and portable subscriber units provides an important means for the base stations to determine the reliability of a message transaction. Digital error detection techniques have worked well for prior art systems which use frequency modulation (FM) for digital message communication. Such messages are typically numeric or alpha-numeric messages. With today's technology improvements in speech algorithms and microcomputer processing, the demand for new services which require higher data rates makes standard FM modulation no longer suitable. To resolve this problem, other prior art systems have developed modulation signaling formats, such as mixed signaling systems which use both FM and linear modulation schemes.
Base stations which deliver mixed digital and analog messages to portable subscriber units can no longer rely on digital error detection techniques to determine if the message transaction is reliable. These systems still utilize conventional error detection or error correction techniques for the digital message portion. This, however, only allows the portable subscriber unit to verify that the digital message portion has been received reliably without any indication of the state of the analog message portion.
Thus, what is needed is a method and apparatus in a radio communication system for implementing a received signal quality measurement. In particular, a method and apparatus is needed that can determine the signal quality level of the analog message portion.